The present invention relates generally to a passive restraint system. More specifically, the present invention relates to a gas generating composition which, unlike the current anhydrous 5-aminotetrazole, hereinafter referred to as "5-AT", based gas generating compositions used to inflate passive restraint systems, is devoid of metal oxides.
Gas generating compositions are extremely useful in the automotive passive restraint (air bag) industry, although other uses, such as commercial or military aircraft applications, are contemplated for such gas generating compositions. Today, most, if not all, new automobiles are equipped with single or multiple air bags to protect the driver and passengers. In the operation of air bags, sufficient gas must be generated to inflate the device in a fraction of a second. The air bag must fully inflate between the time that the automobile is impacted in a collision, and the time the driver would otherwise be thrust forward against the steering wheel or a passenger against the dashboard or sideways against the door of a vehicle or aircraft. Subsequently, nearly instantaneous gas generation is required.
There are a number of mandated design specifications of automobile manufacturers and other agencies that must be adhered to in the preparation of gas generating compositions. One such required specification is that the composition produce gas at a specific rate. The automobile manufacturers require that the gas be generated at a sufficiently and reasonably low temperature so that the occupants of the involved automobile are not burned upon impacting an inflated air bag. Inconsistent ballistic output is a major problem with all pyrotechnic inflators. Accordingly, a need exists for a formulation that minimizes the ballistic variability, prevents the production of excessive heat and maintains an adequate burn rate.
Another specification required by the automobile manufacturing industry is that the gas generating composition strictly limits the generation of toxic gases or solids such as carbon monoxide, nitrogen oxide, sulphur oxide, and hydrogen sulfide. Another related design requirement is that the gas generate composition produces a limited quantity of particulate materials, which can interfere with the operation of the passive restraint system, create an inhalation hazard, irritate the skin and eyes, or present a hazardous solid waste that must be disposed of in an environmentally safe manner. Sodium azide is one such hazardous constituent of gas generating compositions that is currently being phased out by the industry due to its high toxicity as taught in U.S. Pat. No. 6,661,261 to Ramaswamy, et al. and U.S. Pat. No. 5,516,377 to Highsmith, et al. Further, the use of sodium azide (or other azides) results in extra expense and risk in manufacture of gas generate due to the extreme toxicity of azides.
It has also been found that the non-azide propellant technologies are costly to manufacture, and have performance problems such as high burn temperature, undesirable trace effluent values and inconsistent ballistic output. High burn temperatures are undesirable because the gas requires more cooling to maintain gas temperatures. Cooling of the gas is typically performed by the inflator filtration system. Increasing the mass of the filter is a common method of cooling the gas. A disadvantage of increasing the mass of the filter is that it also increases material costs. Cool gas temperatures, however, are required to prevent the air bag and subsequently the occupant of the automobile or aircraft from burning. 5-AT is a desirable alternative to using sodium azide in a gas generating composition as taught in U.S. Pat. No. 5,500,059 to Lund, et al.
It is preferable, in any event, to have a gas generating composition which produces more gas and less solids. The nongaseous fraction of the gas generant products must be contained or filtered to provide a clean inflating gas. It is also desirable that when the composition produces particulates, the majority of these particulates are filterable, solid slag. Solid slag is an insoluble metallic particulate that can be easily filtered, preventing the airborne reaction products from escaping into the surrounding environment during and after air bag deployment. Filtration, therefore, serves a function that limits the dissipation of potentially harmful dust in the vicinity of the spent air bag, which could otherwise cause secondary effects to the passengers and others in the vicinity such as eye, lung, and mucous membrane irritation.
Currently available 5-AT based gas generating compositions form a minimum of water soluble products at combustion of the gas generant. U.S. Pat. Nos. 5,500,059 to Lund, et al.; 5,661,261 to Ramaswamy, et al.; 5,516,377 to Highsmith, et al.; 5,501,823 to Lund, et al.; 5,472,647 to Blau, et al.; and 5,139,588 to Poole, all teach the use of transition metal oxides to preferentially form solid slags.
For example, U.S. Pat. No. 5,500,059 to Lund, et al. teaches that copper oxide is a preferred oxidizer in 5-AT based gas generating composition. U.S. Pat. No. 5,139,588 to Poole teaches the use of transition metal oxides and other metal oxides having high melting points in 5-AT based gas generating compositions to function as high temperature slag forming material.
A slag former may be optionally incorporated in the gas generant in order to facilitate the formation of solid particles that may then be filtered from the gas stream. A convenient method of incorporating a slag former into the gas generant is by utilizing an oxidizer or a fuel which also serves in a dual capacity as a slag former.
The current 5-AT based gas generating compositions which also contain metal oxides tend to produce a predominantly insoluble slag at combustion of the gas generant. A predominantly insoluble slag must be filtered with wire mesh due to its harmful side effects. One of the many disadvantages with the use of metal oxides is that if an insoluble slag is not properly filtered for whatever reason, an occupant of the vehicle would likely experience a harmful gritty blast of dust-like particulates at the deployment of the air bag.
On the other hand, if harmless water soluble products are produced at combustion and not filtered properly, the occupant would instead experience a more mild, saline-like spray of substance at air bag deployment. Accordingly, a need exists for a gas generating composition that reduces the levels of harmful insoluble particulates at combustion, resulting in a safer air bag inflator.
Another problem exists at the storage and pelletization stages for current 5-AT based gas generating compositions that contain metal oxides. Metal oxides, such as the preferred oxidizers, copper and iron oxides, cause the tooling used during the pelletization process to frequently be rendered inoperable due to the "gumming up" and abrasive damage to the tooling at the pellet formation stage. Additionally, the storage of these metal oxides creates a considerable housekeeping problem at the manufacturing plant for the gas generating compound. This severe housekeeping problem, due to the powdery and easily disturbed quality of the metal oxide material, is compounded by the potential explosion risk created by airborne metal oxide dust. Safety is a primary concern in the manufacture of gas generating compositions. Metal oxides also create potential environmental concerns due to inhalation hazards, disposal difficulties and potentially toxic effects. Therefore, the elimination of metal oxides has the potential to significantly reduce the safety risks involved in the manufacture of gas generating compositions.
Additionally, metal oxides introduce a source of impurities into the gas generating composition. Metal oxides often contain impurities that cannot be economically separated. These impurities often alter the combustion characteristics of the gas generating compositions in unpredictable ways. Inconsistent combustion is an undesirable quality for any gas generating composition. Elimination of metal oxides results in greater consistency in the combustion products from the gas generating composition.